1. Field of the Invention
The present invention relates to an electric circuit operating on a single-phase power supply system for controlling the heat output of at least two heating resistances which are electrically connected in parallel and are installed in electric household appliances, particularly toasters, wherein the control is being effected in compliance with flicker standards and without feeding interference back into the power supply system. A switch is connected in series with each heating resistance, wherein the switches are activated by means of an electronic energizing arrangement.
2. Description of the Related Art
If two or a plurality of heating elements are arranged in a network composed of heating resistances and are connected in series or in parallel, it may be desirable, for example, in the case of a toaster, to distribute the maximum available heating power between the heating elements optionally either evenly or symmetrically, on the one hand, or unevenly or asymmetrically, on the other hand. This is because the sides of a piece of bread or other substance to be toasted my differ with respect to their properties.
If all the heating elements have identical connected load values, an even distribution of the heating power is automatically obtained when they are connected in series or in parallel. However, when it is desirable for special heating purposes to reduce the output of one or more of the heating elements, this is possible at present only by cyclically repetitive adjustment of the current that flows therethrough by intermittently switching in and out individual heating resistances or all heating resistances. This control can be effected especially by means of phase-angle power control or multicycle-burst power control by utilizing triacs which are connected in series in front of the heating resistances.
Since, in the case of phase-angle power control the first portion of each half-wave is cutoff, the application of this control technique requires as a matter of principle special measures to minimize effects that result in interference being fed back into the power supply system. Since the loads connected to the power supply system are not connected to one another, and since, therefore, the individual current-pause periods will have a statistically random distribution, these current-phase periods must, in the case of many loads, overlap each other in such a way that they cease to be apparent at the electricity generating plant. For this reason, in cases involving relatively high power levels, the current-pause or multicycle-burst control techniques have generally favorable effects on the power supply system. For controlling the power input to high-rating loads, especially those with a certain lag characteristic, such as heating resistances, repetitive cycling by means of the generally preferably multicycle-burst control technique is nevertheless subject to very rigorous restrictions because of the presence of so-called flicker standards (e.g. EN 60555, Part 3). Because of the fact that, for a given supply-system line impedance, the switching of a load will always give rise to fluctuations in the supply-system line voltage, which, in turn, will give rise to noticeable fluctuations in the intensity of the light produced by any illuminating devices connected at the time, narrow limits are set with respect to repetitive cycling of loads because of the existence of flicker standards, i.e., standards that specify permissible cycling rates. In practice, this results in very low permissible cycling rates in cases involving the repetitive cycling of high heating power, as required particularly in household appliances, such as toasters and stoves. For example, in the case of a 1500 W heating system on a 230 V public power supply system, the permissible cycling rate is limited to approximately 11 switching operations per minute (calculated from the percentage voltage drop as the prescribed standard supply-system line impedance of 0.4+0.25i ohm and read from the permissible flicker curve). This low permissible cycling rate is only of little practical value, especially in cases involving open toaster-type heating elements with very small heating time constants because switching-off results in a rapidly following loss of the shorter-wave length infrared radiation that is significant with respect to the toasting process.
German Patent 39 19 452 discloses a toaster which has two heating resistances connected in parallel. Each heating resistance is preceded by a switching transistor connected in series. The switching transistors are activated by an electronic control device. The two heating resistances are located on both sides of a toasting slot which is especially intended for receiving slices of bread. Appropriate switching means are provided for the purpose of switching the heating resistances in such a way that one of two alternative modes of operation can be selected i.e., the supply of a pulsating direct current to only one heating resistance at a time, or to both heating resistances simultaneously. This toaster makes possible an asymmetrical operation in which the available heating power is distributed between the two heating elements in the proportion of 100% to 0 percent. Circuits of this type, i.e. for half-wave operation of appliances with power ratings exceeding 200 W, do not comply with relevant standards and, therefore, the use of such circuits is prohibited. Moreover, even if circuits of this type were permitted to be connected to the public power supply system, it would be necessary to use very expensive bipolar transistors with a high blocking voltage and with an ability to conduct high forward or on-state currents.
German Patent 36 01 555 discloses a control arrangement for the stepwise switching of the power input to an electric water heater of the continuous-flow type in dependence on the demanded output, wherein the heater has a parallel network of heating resistances which are installed in the water flow duct of the heater and through which patterns of power supply half-waves representing different power levels are conducted either cumulatively or alternately, and wherein these patterns are stored in a memory device. In addition to providing finely graduated power control by means of the multicycle-burst control technique, this arrangement is capable of producing a very substantial reduction in the occurrence of supply-line voltage fluctuations due to switching operations and, in particular, a very substantial reduction in the flickering of lighting units connected to the power supply system. However, because of the fact that sets of power supply half-waves are being switched, special measures have to be taken in order to provide a reliable guarantee that no direct-current component occurs in the time-averaged load current, i.e., that exactly equal numbers of positive and negative power supply half-waves are switched through.
U.S. Pat. No. 5,088,389 discloses a circuit arrangement of the above-described type for controlling the heat output of two heating resistances for a toaster connected in parallel, wherein a triac, controlled by the power supply zero voltage, is connected in series immediately before each heating resistance and the heating power that is input either to only one heating resistance or to both heating resistances is adjusted, either asymmetrically or symmetrically, by repetitive cycling accomplished by means of the multicycle-burst control technique. As the heat-holding timer cycles repetitively, cyclic switching-in and switching-out of both heating elements together is achieved, and the power is thereby reduced to a level that is intended to keep the toasted contents of the toaster warm. This control arrangement has the disadvantage that the mode of operation is based on the assumption of a constant heat output over the duration of a toasting cycle, and that no measures are taken to perform the repetitive cycling in compliance with flicker standards and without feeding interference back into the power supply system.